By using conventional methods the products to be frozen in a conventional freezing tunnel, having at least two or more freezing conveyors supported therein, are fed onto the top freezing conveyor which conveys the products through the freezing tunnel in a continuous movement. On this freezing conveyor only the surface of the products gets to be frozen. The products, though, hardly become firm enough in shape to be moved undeformed onto the next freezing conveyor where the item is fully frozen. Finally the product is delivered from the end of the last freezing conveyor for further processing.
A U.S. Pat. No. 3,708,995 describes a freezing tunnel of the above-mentioned conventional type with a multiple run conveyor, which is formed by three layers of endless conveyor belt, where each conveyor belt moves in the opposite direction compared to the conveyor belt next in row below. In said freezing tunnel the products to be frozen are dropped from the one freezing conveyor belt to the other. The total dwell-time of the products in the freezing tunnel increases with the number of freezing conveyors used and thereby the efficiency per area occupied by the freezing tunnel in the field of general freezing of foodstuff.
The most important factor that limits the efficiency of conventional freezing tunnels in the field of freezing delicate individual items of food, where appearance and shape of the product is important, is the length of time the individual items can dwell in the tunnel free of each other, exposed to the cold flow of air that freezes them without being subjected to any handling that will deform them.
In conventional freezing tunnels such as the one described in U.S. Pat. No. 3,708,995 the time it takes for the products to gain their frozen shape and become sufficiently firm to allow their transfer without damaging their form between the top freezing conveyor and lower freezing conveyors is limited by the area and speed of the top freezing conveyor.
Thus, the main factors that can be utilized to control the surface freezing dwell time of the product on the top freezing conveyor for safe transfer between the freezing conveyors are the size and speed of the top freezing conveyor belt.
Therefore, when the efficiency of the conventional freezing tunnel is under consideration concerning the quick freezing of delicate foodstuff, the actual number of freezing conveyors is irrelevant if the product can not be transferred undamaged and undeformed between the conveyors since it is the surface freezing dwell time of the product on the first conveyor which matters.
Thus it is the area and the speed of the first and topmost freezing conveyor belt which determines the efficiency of conventional freezing tunnels in the field of freezing of delicate products.
For example, a fish fillet to be frozen in a freezing tunnel will need a certain freezing dwell time to reach the firmness needed to stand the handling that accompanies being moved between freezing conveyors. If the fillet is not sufficiently firm in shape to be moved between the conveyors its shape will be altered and deformed when moved between the conveyors in a conventional freezing tunnel. The fillet will therefore take on a deformed shape in its fully frozen condition on the lower freezing conveyors and--thereby, as a sales product--become unsuitable merchandise according to the demands made in the market for frozen fish fillets.
For these reasons conventional tunnel freezers can take up a lot of space, depending on the desired size of the topmost freezing conveyor belt. That can become a major problem when it comes to tunnel freezing delicate products on board ships or in a factory where available space is limited and of the utmost importance for a producer who wants to achieve acceptable productivity.
The extreme cost of building and running large tunnel freezing rooms is considerable and can be decisive in the feasibility of using tunnel freezing as a production method, and the profit achieved by the added value gained by this method of freezing delicate individual items of food can be lost.